close files we open, but forgot to close

This fixes all the obvious `ResourceWarning`s seen in the tests

beetsplug/lastgenre/__init__.py

@@ -141,8 +141,8 @@ class LastGenrePlugin(plugins.BeetsPlugin):
             c14n_filename = C14N_TREE
         if c14n_filename:
             c14n_filename = normpath(c14n_filename)
-            genres_file = codecs.open(c14n_filename, 'r', encoding='utf-8')
-            genres_tree = yaml.load(genres_file)
+            with codecs.open(c14n_filename, 'r', encoding='utf-8') as f:
+                genres_tree = yaml.load(f)
             flatten_tree(genres_tree, [], self.c14n_branches)
 
     @property

test/test_mediafile_edge.py

@@ -87,8 +87,9 @@ class EdgeTest(unittest.TestCase):
         # such aren't recognized by imghdr. Ensure that this still works thanks
         # to our own follow up mimetype detection based on
         # https://github.com/file/file/blob/master/magic/Magdir/jpeg#L12
-        f = open(os.path.join(_common.RSRC, b'only-magic-bytes.jpg'), 'rb')
-        jpg_data = f.read()
+        magic_bytes_file = os.path.join(_common.RSRC, b'only-magic-bytes.jpg')
+        with open(magic_bytes_file, 'rb') as f:
+            jpg_data = f.read()
         self.assertEqual(
             beets.mediafile._image_mime_type(jpg_data),
             'image/jpeg')

test/test_play.py

@@ -88,10 +88,11 @@ class PlayPluginTest(unittest.TestCase, TestHelper):
         self.run_command('play', '-a', 'nice')
 
         open_mock.assert_called_once_with(ANY, open_anything())
-        playlist = open(open_mock.call_args[0][0][0], 'rb')
+        with open(open_mock.call_args[0][0][0], 'rb') as f:
+            playlist = f.read().decode('utf-8')
         self.assertEqual(u'{}\n'.format(
             os.path.dirname(self.item.path.decode('utf-8'))),
-            playlist.read().decode('utf-8'))
+            playlist)
 
     def test_raw(self, open_mock):
         self.config['play']['raw'] = True